Smog test &"Check Engine Soon" light on 2001 Chevy Blazer 2 door?

My sister did not put my gas cap on tight enough, and it caused my check engine soon light came on. I wanted to get it smog tested, but was told because of this light, it would fail. Previous to this I had a 2000 ford ranger that did the same thing. I took it to a certified mechanic who did me a favor and just unhooked the battery cables to make the light go off. He charged me $10, and didnt even have to do a diagnostic on it. He did it right in the parking lot as a favor.A smog guy told me it would cost me $65 for him to hook up a cable to make the light go off, and additional $60 for the smog test, or it would fail. He said if i unhooked the cable myself, it would erase the computer. What does that mean "rease the computer", what if i had to replace the battery, wouldnt that be the same ? All i want the car to do is make the light go out, and be able to p the smog when i need. He said if i did the battery i would have to drive the vehicle at least 100 miles, meaning what ?